Changed behavior:
Write metadata only if the uncompressed version has not been written, has changed, or if
compressed versions are missing (if requested). Not only is re-saving unmodified metadata not ideal, it also forces a new version of the release file if compressed metadata is rewritten. gzip attaches a timestamp to compressed files, which may cause compressed content to generate different digests if independently written.
Update libtuf.py comments.
Minor edit to docstrings of previous commit that addressed interposition issues.
Move 'tuf.client.updater.Updater.refresh()' call in interposition to its updater's __init__(). The refresh() was previously executed on every target request.
Add tuf.interposition.refresh() for integrations that may require a manual refresh of top-level metadata (rare).
